As wafers in use for high performance devices, SOI wafers, each of which is obtained by bonding a bond wafer to a base wafer, have been used. These SOI wafers can be manufactured by the following method, for example. Two mirror-polished silicon wafers (a bond wafer and a base wafer) are prepared. An oxide film is formed on at least one of the wafers. These wafers are bonded together and then subjected to a heat treatment at 200 to 1200° C. to enhance bonding strength. The wafer on which devices are to be fabricated (the bond wafer) is then thinned up to a desired thickness by grinding and polishing, and an SOI wafer having an SOI layer formed therein can be thereby manufactured.
In manufacture of bonded wafers, there is another method of bonding together silicon wafers directly without being bonded through the oxide film. As the base wafer, an insulating wafer such as a quartz, silicon carbide, or alumina wafer may be used.
A surface grinding process and a surface mirror polishing process are generally performed to thin the bond wafer after bonding the bond wafer to the base wafer. When the final thickness of the bond wafer is 50 μm, for example, the surface mirror polishing process is performed with a polishing removal of 10 to 20 μm, and the thickness of the bond wafer after grinding is therefore 60 to 70 μm.
The outer circumference of the bond wafer and base wafer is usually chamfered. In the case of a general chamfer shape of a chamfer width of 350 μm, a chamfer angle of 18°, and a circular section diameter of 280 μm at its edge in a wafer having a thickness of 775 μm and a diameter of 300 mm, for example, the sectional shape of the outer circumference portion of the thinned bond wafer has a very sharp tip angle (an explementary angle) of approximately 20° in a state where the outer circumference portion of the bond wafer is not bonded to the base wafer. In surface grinding process and the subsequent processes, therefore there has been a problem in that the outer circumference portion of the bond wafer frequently chips and the bond wafer is delaminated due to the chipping as a starting point.
To inhibit such chipping at the outer circumference portion of the bond wafer, Patent Document 1 proposes a method of grinding the outer circumference portion of a bond wafer. This method includes a step of grinding the bond wafer so as to remove at least one part of the chamfered portion of a bonded interface side with respect to an outer circumference side of a predetermined width with which the outer circumference portion is to be ground, and a step of grinding the bond wafer up to a thickness of such a degree that a base wafer is not damaged with respect to the other inner circumference side of the predetermined width with which the outer circumference portion is to be ground.
Patent Document 2 proposes a method for performing a thinning process on a bond wafer after grinding to remove the exterior of a straight line that connects a point on the periphery of the chamfered portion of a base wafer and a point on the bonding surface between the bond wafer and the base wafer.
Patent Document 3 proposes a method of thinning a bond wafer by surface grinding after a bonding step and chamfering its outer circumference portion so that the chamfer angle falls in the range of 30° to 60°.